The protozoon Hepatozoon canis and the rickettsia Ehrlichia canis are tick-borne pathogens,
transmitted by Rhipicephalus sanguineus, which cause canine hepatozoonosis and canine
monocytic ehrlichiosis, respectively. Co-infection of the same host monocytes with H. canis
and E. canis confirmed by molecular characterization of the infecting agents and quantitative
assessment of co-infected cells is described for the first time in three naturally-infected
dogs. Blood smear evaluation indicated that at least 50% of the leukocytes infected with H.
canis gamonts contained E. canis morulae. Co-infection of the same host cell demonstrated
in this report suggests that infection with one pathogen may permit or enhance invasion
or prolonged cellular survival of the other.